1. Field of the Invention
The present invention relates to a motor drive apparatus that drives a brushless motor.
2. Description of the Related Art
For products powered by a brushless motor, there has been a growing demand for vibration reduction, noise reduction, torque ripple reduction, and so on. As a brushless motor drive method meeting this demand, a sine wave drive system that drives a brushless motor by supplying a sinusoidal drive current to a drive winding of the brushless motor (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. H07-250492).
FIG. 3 is a timing chart of drive waveforms for a brushless motor based on a common sine wave drive system, and the drive waveforms are based on the assumption that a three-phase four-pole brushless DC motor is driven. Among hall signals E1, E2, and E3 which are output signals from three magnetic pole detecting elements (not shown), only the hall signal E1 is used, and a hall interrupt signal E4 is generated. Here, one period of the hall signal E1 is a period over which the north pole and the south pole of a rotor magnet are detected, and hence the hall interrupt signal E4 is output at intervals of one period of the hall signal E1. The hall interrupt signal E4 is divided in 24 by a divided period computing unit (not shown), and a Sin θ interrupt signal E5 is output.
In synchronization with the Sin θ interrupt signal E5, 24 sine wave amplitude values are supplied to a DA converter, and a DAC output signal E6 which is a one-phase sine wave drive signal is output. A sine wave amplitude value which lags by an electric angle of 60 degrees and a sine wave amplitude value which lags by an electric angle of 120 degrees are supplied to the DA converter, and a DAC output signal E7 and a DAC output signal E8 are output. Thus, sine wave drive signals less affected by variations in the locations at which magnetic pole detecting elements are placed can be output.
However, in an arrangement having a rotor magnet with four or more poles, even when the rotor magnet is rotating at a uniform speed, variations in the period of a hall signal detected by a magnetic pole detecting element are caused by lack of accuracy of division in magnetization of the rotor magnet. For this reason, in the method in which one period of a sine wave drive signal is output from one period of a magnetic pole detecting element, variations in the period of a sine wave drive signal occur. Moreover, when resolution for sine wave amplitude values is low, variations in sine wave amplitude values increase with each period of a magnetic pole detecting element, which causes torque ripples to unfavourably occur.
To solve the above described problems, it is necessary to increase the accuracy of division in magnetization of the rotor magnet and the resolution of sine wave amplitude values, but to satisfy this requirement, the arrangement of a motor drive apparatus has to be complicated, resulting in an increase in the cost of the motor and the motor drive apparatus. Moreover, because the motor drive apparatus is required to have an arrangement suitable for the number of magnetic poles of the rotor magnet, the motor drive apparatus cannot be made versatile.